Air Pollution as a Climate Forcing: A Workshop

Summaries

Summary E. Health Effects

Daniel Greenbaum

Background —
Much known about the health effects of air pollution.
Numerous epidemiologic, human clinical, and laboratory studies in North America and Europe have identified a number of health effects associated with ambient air pollution. In addition, studies in the developing world have identified health effects for people exposed to intense indoor pollution, especially from low efficiency cooking stoves using a variety of biomass fuels. Although there continue to be uncertainties concerning each of these types of effects, there is consensus that air pollution is linked to a number of effects, e.g.:

exacerbation of respiratory and cardiovascular disease, and premature mortality associated with exposure to ambient pollution,

enhanced infectious disease, blindness, and respiratory effects from exposure to indoor air pollution, and

emerging evidence of immune system and developmental effects.

In general, the effects are smaller than those attributed to
several other causes (e.g. smoking). However, recent
epidemiologic evidence in the United States (1) suggests that
the widespread exposures, and the likely magnitude of effects,
would result in these effects being measurable on a global scale.

Effects of Different Ambient Pollutants.
Although there is general consensus that pollution is linked to health effects, the studies to date have found different effects attributable to different ambient air pollutants, and with differing degrees of magnitude for those effects (2,3). There are also differences in the degree to which ambient air pollutants might directly affect climate change: some pollutants for which there are well-understood health effects (e.g. sulfur dioxide and lead) have not been linked directly to climate forcing, while others (e.g. aerosols and ozone) have been more directly linked.

Figure 1 summarizes in general form the effects that have been attributed to ambient air pollutants that have also been associated with climate forcing. Among those effects ascribed to these pollutants are:

Effects of Indoor Pollutants.
In addition to exposures to ambient pollution, populations are
exposed to a variety of indoor air pollutants as a result of
cooking, smoking, and other activities. In most instances,
these exposures, which are a combination of ambient and
internally-generated emissions, are significantly larger than
those experienced outside the home. This is especially true in
the developing world, where the primary sources of cooking
energy and heat is the inefficient combustion of wood, poor
quality coal, and biomass such as animal dung. In these
interior settings, there is evidence of significant health
effects due to the exposure, especially among women, who are the
operators and users of the cooking combustion sources, and their
children. Among other potential health effects, exposure to
these largely carbonaceous aerosol and gaseous pollutants has
been found most strongly to be associated with acute respiratory
infections in young children and chronic obstructive pulmonary
disease and lung cancer in adult women. There is also moderate
evidence of enhancement of tuberculosis, cataracts and increased
blindness, and asthma attacks, and emerging evidence of enhanced
promotion of infectious disease (4).

Estimating the Public Health Impact.
Two approaches for estimating the public health impact were
described during the meeting.

The first is efforts now underway under the auspices of the
World Health Organization to estimate the Global Burden of
Disease (GBD) for a variety of preventable exposures, including
indoor and outdoor pollution (5). Based on synthesis of
currently available epidemiology and economics-based estimates
of ambient air pollution levels worldwide, that analysis has
yielded a preliminary estimate, made using PM as an indicator of
air pollution, that 1%-4% of mortality is attributable to
exposure to particulate matter air pollution (PM). Although
data is available on a variety of other health effects (e.g.
respiratory hospitalization and ozone exposure) the GBD effort
has not yet made comparable global estimates of impact, nor of
other measures of impact (e.g. quality adjusted life years or
QALYs). Although these estimates of effect have been capable of
linking these effects to specific components of the complex
mixture that makes up PM. Comparable efforts are underway to
estimate the Global Burden of disease from indoor air pollution.

The second approach has been to estimate the benefits of
specific interventions, for example in China and Chile. In
these analyses, substantial benefits have been estimated, again
primarily based on estimates of avoided premature mortality
associated with reductions in exposure to PM. To date, these
estimates have been based on extrapolation from effects
documented in two studies of long term exposure and mortality in
the United States (6,7) and to some extent using local health
studies. Although these estimates use care in estimating
effects in developing countries based on United States data,
they would undoubtedly be improved with enhanced availability of
epidemiologic studies in the developing countries themselves.

What Do We Need To Learn?
Based on the presentations and discussions at the workshop,
several key needs emerged for future research:

Better Understanding of Effects of Different PM Components
and Sources.
It is clear that one of the principal needs to
better examine the relative co-benefits and disbenefits of
reducing ambient pollutants from both a climate and health
basis will be to better determine the relative toxicity of the
different air pollution components. While there are today
relatively well documented effects of several components of the
mixture — e.g., traffic emissions and diesel particulate
matter, sulfates, and ozone — it is difficult in the current
literature to ascribe stronger effects to one or another of
these pollutants. This will be a significant challenge, in
part because of the significant collinearity of exposure, and
in part due to the diversity of effects and of potentially
susceptible populations. Even as initial actions to reduce
exposure move forward (e.g. tougher standards in the United
States, Europe, and Japan for heavy-duty diesel engines), this
set of constituents will need systematic epidemiology and
toxicology studies over the next decade to identify relative
toxicity and inform future control strategies (8).

Improved Concentration-Response Data in Developing World.
Eighty percent of current time series studies of air pollution
effects, and 100% of cohort studies, have been conducted in
Europe and North America. Although there are today reasonable
methods of extrapolating from these studies to the developing
world, the accuracy of estimated effects, and the willingness
of local political leaders to accept the results, will be
enhanced with increased use of local studies.

Estimates of Morbidity Impact for PM and Ozone.
To date,
most efforts to estimate the health benefits of reducing air
pollution have been estimates of the reductions of premature
mortality likely to result from pollution reductions. Although
these effects are significant, and likely carry the largest
estimates of economic benefits, it is also important to
estimate the likely benefits from reduced hospitalizations and
reduced loss of work days as a result of pollution reductions.
This is feasible given existing health and exposure data, but
requires time and resources in order to be implemented. It is
important to note that these impacts, once estimated, are
likely to be large in number, but not in economic value (9).

Better understanding of synergistic effects. Populations
are subjected simultaneously to multiple pollutant exposures.
There is the potential of one pollutant to activate and/or
facilitate the effects of others (e.g., PM as the "escalator"
for the introduction of metals, organics, and gases into the
deep lung).

How Do the Atmospheric and Health Communities Need To Work Together?
The successful addressing of these questions will require
enhanced collaboration among the health and atmospheric
communities. This will require (a) developing common
understandings of key facts, e.g., what is black carbon and how
best to represent it in health studies, (b) identifying
atmospheric data sources that could sharpen future health
analyses, e.g., atmospheric sulfate, BC data sets that might be
applied in health studies to help distinguish effects, (c) the
bringing together of source-specific climate forcing data with
source-specific health analysis, and (d) developing better means
to communicate results — in an understandable fashion — to
decision makers in the developed and developing worlds.

Summary and Conclusions.
There is evidence of health effects from both aerosol and
gaseous pollutants to which populations are exposed both
outdoors and indoors. To date there is evidence of effects from
most components (constituents) of the aerosol mixture, and it
has not been possible to disentangle the effects of one
component from the others. Estimates of mortality effects due
to worldwide and local PM pollution have been made, suggesting
substantial effects on premature mortality (1%-4% of all
mortality depending on region). Estimates of morbidity effects
from PM and ozone have yet to be made. Looking forward, there
is a need for improved efforts to distinguish the effects of
different components of the PM aerosol, increased number of
studies of effects in developing countries, and better
understanding of the effects of aerosol from biomass combustion.
Enhanced cooperation among atmospheric and health scientists
will facilitate the timely addressing of these remaining questions.

References

National Research Council, Committee on Research
Priorities for Airborne Particulate Matter.
Research priorities for airborne particulate matter: I.
Immediate priorities and a long-range research portfolio.
Washington, DC: National Academy Press, 1998.